feat: implement Story 4.2 - New Participant Registration

Implement participant registration with the following features:

- POST handler for /exchange/<slug>/register
- Create Participant record with lowercased email
- Generate magic token and send confirmation email
- Redirect to success page after registration
- Rate limiting: 10 registrations per hour per IP
- Validation for exchange state (must be registration_open)
- Form validation for required fields (name, email)
- Email format validation
- Optional fields support (gift_ideas, reminder_enabled)

Also includes:
- Registration success page template
- 429 error handling template
- Flash message support in base template
- Test config update for email service dev mode
- Comprehensive test suite with 8 tests

All tests passing (86 total), 91% coverage maintained.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/app.py

@@ -112,10 +112,7 @@ def register_error_handlers(app: Flask) -> None:
     @app.errorhandler(429)
     def rate_limit_error(_error):
         """Handle 429 Too Many Requests errors."""
-        from flask import flash, redirect, request
-
-        flash("Too many attempts. Please try again later.", "error")
-        return redirect(request.referrer or "/"), 429
+        return render_template("errors/429.html"), 429
 
 
 def register_setup_check(app: Flask) -> None:
@@ -140,6 +137,7 @@ def register_setup_check(app: Flask) -> None:
             "static",
             "health",
             "participant.register",
+            "participant.register_success",
             "participant.request_access",
             "participant.magic_login",
             "participant.dashboard",